1. Field of the Invention
The present invention relates to a solid electrolytic capacitor. The invention also relates to an electric circuit using a solid electrolytic capacitor.
2. Description of the Related Art
As an example of solid electrolytic capacitor, there exists a so-called three-terminal solid electrolytic capacitor. (See JP-A 2003-158042, for example.) In such a solid electrolytic capacitor, the circuit current flows from an input anode terminal through an anode body toward an output anode terminal for realizing low impedance in a wide frequency range.
For example, JP-A 2003-163137 discloses a solid electrolytic capacitor which is excellent in terms of high capacitance, low ESL (equivalent series inductance) and low ESR (equivalent series resistance). The capacitor includes an anode body formed of a porous sintered body made of so-called “valve metal” such as tantalum or niobium.
The solid electrolytic capacitor having the above-described structure is used as connected between an electronic device such as a CPU and a power source circuit as a bypass capacitor, for example. Recently, in accordance with an increase in the operation speed and digitalization of electronic devices, a power source system with high stability and high response speed is demanded. Therefore, also with respect to a solid electrolytic capacitor used for noise cancellation and stabilization of a power system, excellent noise cancellation ability for a wide frequency range and high-speed response in power supply are required. Further, in accordance with high current power supply, high capacitance and high tolerable power loss are also demanded strongly.
Generally, the frequency characteristics of the impedance Z of a solid electrolytic capacitor is determined based on the following formula:Z=√{square root over ((R2+(1/ωC−ωL)2))}(ω: 2πf (f: Frequency), C: Capacitance, R: Resistance, L: Inductance))
As will be understood from the above formula, in a frequency range lower than the self resonant point, 1/ωC becomes dominant, so that the impedance can be reduced by increasing the capacitance C. In a high frequency range around the self resonant point, the resistance R becomes dominant, so that the ESR need be reduced to reduce the impedance. For example, a porous sintered body whose surface area is increased for increasing the capacitance is advantageous to reduce the ESR. As another conventional means for reducing the ESR, the cathode may be made of manganese dioxide or conductive polymer. However, in an ultra high frequency range higher than the self resonant point, ωL becomes dominant, so that the ESL need be reduced to reduce the impedance. Since the ESL increases as the volume of the porous sintered body increases, to reduce the impedance in the ultra high frequency range becomes more difficult as the capacitance of the capacitor is increased.
In the three-terminal solid electrolytic capacitor disclosed in JP-A 2003-158042, the ESL is reduced by the structure in which the circuit current flows through the anode body. Recently, however, an electronic device as the object of power supply often requires high direct current. For example, when the driving current for a HDD (hard disk drive) is included in the circuit current, the increase of the current is considerable. When a high current flows through the anode body of the tree-terminal solid electrolytic capacitor, the amount of heat generated in the solid electrolytic capacitor increases. Particularly, when the anode body comprises a porous sintered body made of a valve metal and an anode wire is partially embedded in the porous sintered body, significant local temperature increase occurs at the junction between the porous sintered body and the anode terminal. Further, the heating of the porous sintered body may cause cracking of the sealing resin covering the porous sintered body, which need be prevented. In this way, conventionally, it is difficult to improve the high frequency characteristics by reducing the ESL while increasing the tolerable power loss in accordance with the increase of the circuit current.